1. Field of the Invention
This invention relates to a method for producing electricity utilizing a renewable resource, in particular, sewage sludge, in an efficient and environmentally responsible manner. More particularly, this invention relates to a multi-stage method for generating electricity from sewage sludge in which the sewage sludge is first anaerobically digested and the digested solids are then gasified.
2. Description of Related Art
To date, the utilization of sludge for the production of electricity has been hindered or has not been economical for a variety of reasons. First, the direct combustion of sewage sludge, in addition to being environmentally restrictive, is very inefficient due to its elevated water content, which exceeds 75 to 90% by weight. Second, because of its elevated water content, it is not economical to dry the sludge before it is used as a fuel for direct combustion. Third, the production of biogas from sewage sludge through the use of anaerobic bio-digesters has not been optimized. That is, digested solids that must eventually be discarded comprise a majority of the original sludge volume. And, finally, the disposal of sewage sludge, or biosolids, in landfills or its utilization for other applications, such as fertilizer or soil amendment, clearly needs to be examined in light of environmental issues relating to pathogens as well as organic and inorganic contaminants. The presence of inorganic and organic contaminants in digested sludge applied to the land clearly poses a potential danger to underground water supplies and eventually to human health.
It is apparent that there is a clear need for a process that not only achieves improvements in sludge utilization efficiency, but also does so in an environmentally safe manner.
Accordingly, it is one object of this invention to provide a method for converting sewage sludge to electricity having improved efficiency over conventional methods for utilizing sludge to produce electricity.
It is another object of this invention to provide a method for converting sewage sludge to electricity which addresses issues of efficiency arising from the substantial amount of water present in the sewage sludge.
It is yet another object of this invention to provide a method for converting sewage sludge to electricity which renders non-carbonaceous or inorganic components of the sludge environmentally benign.
These and other objects of this invention are addressed by a method for generating electricity in which the sewage sludge is first digested in an anaerobic digester forming a gas mixture of methane and carbon dioxide. Single-stage or multi-stage-anaerobic bio-digesters may be employed for this purpose. Water present in the digested sludge is then removed, forming dewatered sludge. The water may be removed by any of a number of known methods, such as filtration, centrifugation, and thermal drying. The dewatered sludge is then gasified in a cyclonic gasifier or other type of gasifier known to those skilled in the art to form a gas mixture comprising carbon monoxide and hydrogen. The gas mixture of methane and carbon dioxide from the anaerobic digester is mixed with the gas mixture comprising carbon monoxide and hydrogen from the gasifier and the resulting mixture is then burned in an apparatus suitable for generating electricity, such as a gas-fired turbine, resulting in the generation of electricity.